1. Field of the Invention
The invention concerns a cable for transmitting information made up of electrical conductors.
2. Description of the Prior Art
Nowadays information is transmitted by electrical cables comprising a large number of conductors and at very high frequencies. The increase in the frequencies and the number of streams of information to be transmitted in parallel on different conductors leads to the need to provide effective isolation between the various electrical conductors grouped together in the same cable. The risk of crosstalk increases with the number of conductors and the frequency. Crosstalk occurs because information transmitted on one conductor is radiated toward another conductor.
Information is usually transmitted on pairs of conductors and to prevent crosstalk, or near-end crosstalk, the pairs are isolated from each other. They can also be shielded.
The most usual solution to the problem of shielding each pair of conductors is to wind a metal or metallized strip in a helix around each pair, the cable being formed by assembling the screened pairs within a common protective sheath. The operation of individually shielding the pairs is slow and difficult. Also, to connect the resulting cable the individual shielding of the pairs must be removed to obtain access to the conductors, which makes connection, which is usually carried out on site, difficult.
To remedy the above drawbacks a cable for transmitting information has already been proposed, in document FR-2 738 947, in which the electrical shielding of the various conductors, in particular the pairs of conductors, comprises a central member with radial fins separating the pairs from each other and providing partial shielding of each pair, and a peripheral shield around the central member and the pairs of conductors, the peripheral shield completing the shielding of each pair. However, it has been found that the fins on the central member act like reinforcing ribs, so that the resulting cable is unsuitable for many applications in which the cable is required to be flexible.
The invention provides a flexible cable that has good isolation between conductors or between pairs of conductors and which is also particularly simple to manufacture and to use.
In a first aspect the invention consists in a cable for transmitting information including a set of conductors or pairs or quads of conductors isolated from each other to limit crosstalk between said conductors, which cable includes a flexible central cord and in which cable said conductors or pairs or quads of conductors are separated by peripheral cords which are also flexible and distributed around the central cord, the assembly being surrounded by a sheath.
The cords are made from an insulative material such as polypropylene, for example.
The central cord and the peripheral cords delimit compartments separating the conductors or the pairs of conductors.
In the preferred embodiment the cords are deformable to constitute a filling of the cable. In this case the cords, which are originally circular in section, are deformed to fill the gaps between the conductors or pairs of conductors. This minimizes the overall size, i.e. the external diameter, of the cable.
The cords are fibers, for example.
The assembly is preferably surrounded by an external shield, such as a polyester tape covered with aluminum, and possibly a metal braid, for example a tinned copper braid. The external shield is covered with a jacket, for example of fireproof polyolefin.
The cords can be metallized to improve the shielding and therefore to increase the attenuation of crosstalk.
A cable of the above kind can be made with the machines usually employed, without it being necessary to modify the machines and without it being necessary to reduce the speed of manufacture. The cords are installed in the same manner as the conductors or pairs of conductors are assembled.
Also, the conductors have the same relative position at practically all times and so the spread in the electrical characteristics of the manufactured cables is limited. The position of the pairs of conductors relative to each other remains stable because of the damping effect of the filling consisting of the cords. Moreover, because of the flexibility of the cords and the simplicity of the structure, the cables are particularly rugged.
It has been found that, despite its simplicity, a structure of the above kind provides excellent crosstalk isolation between the conductors or pairs of conductors, as high as 50 dB at 200 MHz, and this even in the absence of shielding completely surrounding each pair of conductors.
In another aspect, the invention consists of a cable for transmitting information which includes at least one flexible and deformable cord so that, when the peripheral sheath surrounds the conductors (or pairs of conductors), the conductors exert a pressure forming compartments in which the conductors or pairs of conductors are lodged.
The cord can also comprise fibers. It can be metallized to constitute a shield, at the same time as the external shield.
Note that, regardless of the embodiment, compared to conventional cables the longitudinal regularity of the electrical impedance is improved, enabling the manufacture of larger twisted pairs than with conventional cables. The aim of twisting is to regularize the impedance in the longitudinal direction. The invention allows an average lay of 30 mm compared to an average lay, other things being equal, of 20 mm in a conventional cable. Manufacture can therefore be accelerated and the cable is easier to connect.
Other features and advantages of the invention will become apparent from the description of certain embodiments of the invention given with reference to the accompanying drawings.